Multiplexing users over time or over code of spread spectrum are the existing techniques used to better utilize the radio spectrum by allowing multiple users to share the same physical channel. TDMA stands for “Time Division Multiple Access”, while CDMA stands for “Code Division Multiple Access”. Three of the four words in each acronym are identical—“Division Multiple Access”—since each technology essentially achieves the same goal. The goal is to enable more than one participant to carry on a conversation on the same physical channel without causing interference.
Where the two technologies differ is in the manner in which the participants share the common resource. TDMA divides the channel into sequential Time Slices. Each participant of the channel takes turns transmitting and receiving. Therefore, only one participant is using the channel at any given moment, but they use it for short bursts. They then give up the channel momentarily to allow other users to have their turn. This is similar to how a computer with just one processor can run multiple applications apparently simultaneously.
CDMA on the other hand really does let everyone transmit at the same time. What makes it work is a special form of digital modulation called “Spread Spectrum”. This form of modulation takes the participant's packets of bits and spreads them across a very wide channel in a pseudo-random fashion. The “pseudo” is important, since the receiver must be able to undo the randomization in order to collect the bits together in a coherent order.
Some networks are “one to one” and each of the participants in turn transmits to the location where the task of the Master Of the Network (“MON”) is residing.
There are other networks where the connection is “one-to-many”. That is, many are allowed to listen to the message while one is transmitting. This type of network allows participants to travel far out of the MON range. Once all participants keep the Time Slice that was allocated for them, the network is functional and stable.
The similarity in the CDMA system is allocating a code of the spread spectrum rather than Time Slices in TDMA.
This type of network cannot allow a new participant to join the network when far from the MON, as they cannot be allocated a new Time Slice or code.
One known solution for this problem is to allocate spare Time Slices for a new, joining participant. The new participant is given relevant MON regulations regarding the utilization of spare Time Slices/codes. This solution cannot accommodate the arrival of a new participant where all spare Time Slices/codes are already allocated.
Existing technologies for communication between moving participants solves the issue of a distributed MON by each participant having a pre-calculated priority list. The size of the priority list is the number of Time Slices available multiplied by the maximum number of participants allowed. For example:                1. A network with a maximum of 500 participants and 200 Time Slices per second, needs to have a table of: 200 Time Slices×9 bits of user ID×500 participants=900 Kbits        2. A network with a maximum of 1000 participants and 200 Time Slices per second, needs to have a table of: 200 Time Slices×10 bits of user ID×1000 participants=2,000 Kbits        
The size of the table increases linearly related to the increase in the number of participants.
Each participant has to share with the rest of the network's participants its list of network members that are within receiving range of the participant. This leads to large message sizes. For example:                1. A network with a maximum of 500 participants and 200 Time Slices per second, the network message needs to be: 9 bits of user ID×500 participants=4.5 Kbits        2. A network with a maximum of 1000 participants and 200 Time Slices per second, the network message needs to be: 10 bits of user ID×1000 participants=10 Kbits        
The size of the network message increases linearly related to the increase in the number of participants. The network message has to be transmitted over the network by each participant. This size of the message leads to congestion and delay, and impacts on the size of the Time Slices required for each user.
The existing solution for distributed MON is fine for applications with a low number of participants. Increasing the number of participants incurs the penalty of lengthy transmission cycles, high overheads, and so forth, to the stage that this solution is no longer practical.